Giz Explains: How Did We Get To Pluto So Fast?

On July 14, the New Horizons spacecraft will make history when it sails past Pluto, formerly known as the ninth planet. Even more incredible is how fast we got there. The spacecraft travelled 4.8 billion kilometres in nine and a half years. That's about 1.6 million kilometres a day for almost 10 years. How the heck did we do it?

Size and timing mattered a lot. So did Jupiter. Since the early days of the Space Age, we've learned to exploit nature to shave years off our interplanetary journeys. Here's how humanity's very first Pluto mission made the long haul at breakneck speed.

The Fastest Launch In History

On the afternoon of January 19, 2006, a piano-sized spacecraft weighing 470kg roared into the sky aboard an Atlas V rocket. Separating from its solid fuel-kick motor after just 45 minutes, New Horizons was flung away from the Earth at solar system escape velocity, roughly 58,000km/h per hour. It was the highest speed at which a spacecraft has ever escaped Earth's gravity well, besting the previous launch speed record (52,000km/h) set by the Pioneer 10 probe in 1972.

How did we get New Horizons off the ground so fast? Basically, size. The compact craft was designed to run light on power and fuel, reserving most of its payload for its seven onboard science instruments. At liftoff, the New Horizons propulsion system included only 77kg of hydrazine propellant. That's a minuscule amount when you consider the craft's journey, but it was intended to be used only for trajectory corrections and spinup/spindown maneuvers.

In other words, New Horizon's 10-year, 4.8 billion kilometre journey would be basically propulsion-free. (Now if that doesn't sound like the best car sales pitch, ever.)

After liftoff, New Horizons received additional velocity boost from Earth's orbital motion around the sun, which is approximately 30km/s tangential to the orbital path. Altogether, then, the spacecraft barreled into the solar system with a heliocentric (sun-relative) speed of nearly 160,000km/h.

The timing of the launch was critical. Based on the orbital position of the Earth, NASA was looking at a short window, from mid-January through early February 2006, in which the New Horizons spacecraft could be launched in order to make a close pass by Jupiter in 2007. And we needed Jupiter big time.

The fastest route between two points on Earth may be a straight line, but when it comes to outer solar system exploration, nothing beats a little cosmic kick from Jupiter's massive gravity well.

Jupiter's Big Gravity Assist

Thanks to its quick start, New Horizons made the 800 million kilometre journey to Jupiter in just over a year, faster than any of the seven previous Jupiter-bound missions. But the sun's gravitational pull is relentless, and by the time New Horizons reached Jupiter in early 2007, it had slowed to (a mere!) 69,000km/h. Jupiter would help New Horizons regain what it had lost.

As it neared the gas giant, New Horizons began to speed up once more, reeled in by Jupiter's prodigious gravity, which also acted to bend the spacecraft's trajectory. On February 28, 2007, the tiny probe made its closest approach to the gas giant and then flung itself away, snagging a bit of Jupiter's momentum in a move that rocket scientists call a 'gravity assist.' Essentially, as New Horizons was dragged into Jupiter's gravitational field, it gained kinetic energy amounting to nearly 15,000km/h worth of speed, increasing its velocity to over 83,000km/h.

To balance the books, Jupiter lost as much kinetic energy as New Horizons gained, causing its orbit around the sun to slow by a small amount. A year on Jupiter today is slightly longer than it was before — all because humans wanted to a good look at Pluto.

The effect of Jupiter's gravity assist can be seen on the graph above, which shows us the spacecraft's heliocentric velocity as a function of distant from the Sun. For comparison, the graph below shows Voyager 2's trajectory across our solar system from 1977 to 1989. As you can see, Voyager 2 used several gravity assists to keep up a quick pace as it toured the planets in our outer solar system. Both spacecraft continued to lose velocity as they headed into the outer solar system, but at a decreasing rate as the Sun's gravity field weakened. (The weakening gravitational pull of the sun also explains why solar system escape velocity decreases as we travel outward.)

Picture: Cmglee, Wikipedia

Voyager's clever route afforded the spacecraft a series of speed boosts while allowing it to collect a trove of planetary science data. Likewise, New Horizon's Jupiter detour sent back a wealth of fascinating intel on our solar system's largest planet, revealing lightning near the poles, the internal structure of volcanoes on Io, and the path of charged particles traversing the length of the gas giant's long magnetic tail.

Oh, and it also shaved three full years off the trip to Pluto.

The Approach and Beyond

For the next seven and a half years, New Horizons sailed quietly across interplanetary space, on the longest leg of its journey. Its last major waypoint before the Pluto approach came on August 25, 2014, when made another record, crossing Neptune's orbit some 4.4 billion kilometres away from the Earth in eight years, eight months. (Coincidentally, NASA's Voyager 2 spacecraft made its much closer Neptune pass 25 years earlier to the day. A cosmic coincidence, perhaps, but one that everybody took as a promising sign of what was to come.)

To put the full trajectory in perspective, here are a few images from NASA:

For the past year, New Horizons has continued to stay the course, losing speed slowly as it approaches Pluto. Still, New Horizons's historic flyby won't exactly be a leisurely stroll. At 11.50am UTC on July 14, the spacecraft will sail past Pluto at a blistering 48,000km/h relative to the dwarf planet's surface.

After New Horizons spends some time exploring Pluto and its three moons, it will hopefully continue on to the Kuiper belt, a vast rim of primordial, icy debris that encircles our solar system. The three promising Kupiter Belt Objects, or KBOs, NASA has identified as targets are all roughly a 1.6 billion kilometres from Pluto, and it will take New Horizons another three to four years to reach any of them. NASA is waiting to get some science back from the primary Pluto mission before making a final decision on the Kuiper Belt extension. In the meanwhile, we're left with the mind-blowing possibility that by 2020, New Horizons could be beaming home data on a cosmic graveyard 6.4 billion kilometres away.

And after the Kuiper belt? New Horizons is expected to eventually join Voyager 1 and 2 as the third human probe to enter interstellar space. Launched nearly 40 years ago, both of the Voyager probes are still in communication with the Earth as they wander about the cosmic hinterlands. It's impossible to say just how far from home any of these probes will get.

But if one thing's clear, it's that New Horizon's astounding journey has only just begun.

Picture: Pluto and its moon Charon, captured by New Horizons on July 8 from a distance of nearly six million kilometres. Via NASA-JHUAPL-SWRI

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